The present invention relates to a calorimeter apparatus and calorimetry process for making highly accurate measurements of power generated by steady state sources, such as radioactive materials.
Calorimeters often are used to assay steady state sources, such as radioactive materials, by measuring the thermal power emitted by the source. To assay a radioactive source, the source is placed in a constant temperature environment and its thermal output is measured by non-destructive techniques, such as power replacement calorimetry. The isotopic mass is computed from the measured power through known watts/gram constants for each of the isotopes.
Calorimetry is the most accurate method of accounting nuclear materials because the accuracy of the measurement is not affected by the size, shape, or form of the material being assayed. For example, the material resulting from the manufacture of nuclear weapons often is found in odd shapes and sizes, making other assaying techniques difficult or impractical. Likewise, beta emitters, such as tritium, which may be packaged in many forms, are very difficult to quantify using chemical or nuclear counting methods.
In order to obtain precise results, however, the calorimeter must control the flow of heat so that no external heat enters the system and no heat escapes from the sample without being detected. As a result, calorimeters typically have a complex arrangement of heat shields which insulate the sample from external heat sources and direct the emitted heat through a desired path. Typically, calorimeters have used water jackets as heat shields due to the desirable insulating and temperature variation control characteristics of water. Water jackets, however, present several problems. For example, water jackets substantially complicate maintenance procedures for the calorimeters. Moreover, mixing water and radioactive sources creates the possibility of dangerous chemical reactions, nuclear criticalities and other problems. Thus, a water leak in a calorimeter can create significant safety hazards to workers in the vicinity of the calorimeter.
Calorimeters must have an opening through which the sample is deposited in the calorimeter for measurements to be taken. Typically, these openings are sealed with a lid of some form. Conventional lids can adversely affect the accuracy of the measurements by allowing air and heat to flow in and out of the test chamber. Very hot or cold samples can dramatically change the air pressure in the test chamber, causing air to xe2x80x9cpuffxe2x80x9d in or out of the calorimeter. Inadvertent exchanges of air such as this can cause significant errors in the measurements taken with the calorimeter due to unaccounted and unmeasured heat losses associated with the exchange.
Power replacement calorimeters measure the heat output of the sample by determining the amount of energy required to hold constant the temperature of the calorimeter once the sample has been introduced. The quality of the measurement depends on knowing when the output signal from the calorimeter is stable and no longer affected by heat sinks or sources from mechanical structures or changes in environmental conditions. Conventional calorimeters rely solely on the calorimeter output signal to determine stability. The calorimeter output signal typically is drawn from a very limited region of the overall structure of the calorimeter. As a result, inaccuracies occur when stability is mistakenly assumed. The shielding, closures and control systems used in conventional calorimeters may cause safety hazards and introduce significant inaccuracies in the measurements taken with those calorimeters.
The present invention is a heat-flow calorimeter made up of a heat-conducting rod with a test chamber affixed to one end and a heat sink affixed to the other. The heat sink is maintained at a constant temperature and the heat liberated or absorbed by the test sample is measured by determining the amount of energy that must be introduced into the system to maintain a constant temperature differential across the length of the heat-conducting rod.
A zero heat transfer envelope is used to insulate the unit and prevent heat from xe2x80x9cleakingxe2x80x9d into or out of the calorimeter. The envelope is made of three heat shield systems. The first shield is thin, highly conductive, and affixed to the measurement chamber. The sample is enclosed in the measurement chamber; thus, allowing the first shield to effectively match the sample temperature. The second shield is relatively massive compared to the first shield and its temperature is matched to that of the first shield. The third shield surrounds the second shield and its temperature is controlled to a constant value; thus, protecting the zero heat transfer envelope from ambient temperature variations.
Each of the shields are heated with distributed electrical resistance heaters rather than a water jacket. This eliminates problems that typically arise from water jacketed heat shields, such as maintenance problems, potentially dangerous interactions between the samples and water, and corrosion. Temperature matching of the shields significantly improves calorimeter sensitivity and accuracy.
A labyrinth-sealed plug, which is made up of alternating layers of foam insulation and metallized plastic film, is used to seal the test chamber. This allows easy removal of the sample while providing a stable barrier to heat exchange. While a xe2x80x9ctightxe2x80x9d seal might allow air to xe2x80x9cpuffxe2x80x9d out of the chamber as a result of heat generated pressure, the multiple layers of the labyrinth seal allows room for the air to move without allowing it to leave the chamber.
The control system includes an automatic stability routine which determines when the calorimeter is stable. The program continually monitors the elements of and sensors distributed throughout the calorimeter to detect any changes or drifts in input or output. This data is used to accurately assess stability. Data from temperature control points and power and cooling devices are analyzed to see that all control points are at the prescribed set points and that the values are not drifting over time. Various approaches, including proportional-integral-derivative (PID), state-space/PID and fuzzy-logic controls have been used to implement this strategy. This type of dynamic control improves the accuracy of the calorimeter.
The calorimeter may have between 5 to 12 control loops. For each of the control loops, the temperature and power data are acquired by a digital multimeter. This data is requested by and transmitted to the control computer. The control computer processes the data through the software control algorithms and stability checks. The control algorithms produce a digital number which is processed through a digital-to-analog converter board. The analog output is fed to linear power supplies which are configured in an operational amplifier mode. The power supply output feeds the heaters and coolers which in turn affect the system temperatures.
The overall accuracy of the new calorimeter is enhanced by the combination of these elements together with other features such as the use of an extremely sensitive heat sink; encapsulation of the entire device, including the heat-conducting rod, with insulation; direct temperature measurements using thermistors; and minimal attach points to the zero heat transfer envelope and heat removal system.
Accordingly, it is an object of the present invention to provide a calorimeter which does not use a water jacket.
A further object of the invention is to provide a closure for a calorimeter that eliminates inadvertent exchanges of air between the test chamber and the outside environment.
Another object of the invention is to provide a calorimeter having a control system which substantially improves the precision and repeatability of measurements.
Other objects, features, and advantages of the present invention will become apparent with reference to the remainder of the written portion and the drawings of this application, which are intended to exemplify and not to limit the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side cross-section of a calorimeter in accordance with the present invention.
FIG. 2 is a chart comparing the baseline repeatability of conventional calorimeters and a calorimeter in accordance with the present invention.
FIG. 3 is a chart comparing the measurement precision of conventional calorimeters and a calorimeter in accordance with the present invention.
FIG. 4 is a chart comparing the baseline repeatability of conventional calorimeters and a calorimeter in accordance with the present invention.